In general computed tomography, tomographic images of an examination object, in particular of a patient, are taken with the aid of absorption measurements of X-rays that penetrate the examination object, a radiation source generally being moved circularly or spirally about the examination object, and a detector, for the most part a multirow detector with a multiplicity of detector elements, measuring the absorption of the radiation upon passage through the examination object on the side opposite the radiation source. For the purpose of tomographic imaging, tomographic slice images or volume data are reconstructed from the measured absorption data of all the measured spatial rays. Very fine absorption differences in objects can be displayed with the aid of these computed tomography images, but zones of similar chemical composition that naturally also have a similar absorption behavior are displayed only with unsatisfactory detail.
It is known, furthermore, that the effect of the phase shift upon passage of a ray through an examination object is substantially stronger than the absorption effect of the material penetrated by the radiation. Such phase shifts are known to be measured by the use of two interferometric gratings. These interferometric measuring methods are referred to, for example, in “X-ray phase imaging with a grating interferometer, T. Weitkamp et al., Aug. 8, 2005/Vol. 12, No. 16/OPTICS EXPRESS”. In the case of this method, an examination object is trans-irradiated by a coherent X-radiation and subsequently guided through a pair of gratings, and the radiation intensity is measured directly after the second grating.
The first grating produces an interference pattern that images a Moiré pattern onto the detector lying therebehind with the aid of the second grating. If the second grating is slightly displaced, this likewise results in a displacement of the Moiré pattern, that is to say a change in the spatial intensity in the detector lying therebehind, which can be determined relative to the displacement of the second grating. If the change in intensity is plotted for each detector element of this grating, that is to say for each ray, as a function of the displacement path of the second grating, the phase shift of the respective ray can be determined. The fact that this method requires a very small radiation source is a problem, and therefore cannot be applied in practising computed tomography of relatively large objects, since formation of the interference pattern requires a coherent radiation.
The method shown in the abovenamed document requires a radiation source with an extremely small focus such that a sufficient degree of spatial coherence is present in the radiation used. However, when such a small focus is used there is then, in turn, an insufficient dose rate for examining a relatively large object. However, there is also the possibility of using a monochromatically coherent radiation, for example a synchrotron radiation, as radiation source, but the construction of the CT system is thereby rendered very expensive and so a widespread application is impossible.
This problem can be circumvented by arranging a first absorption grating inside the focus/detector combination in the beam path, directly following the focus. The alignment of the grating lines is in this case parallel to the grating lines of the interference grating following the examination object.
The slits of the first grating produce a field of individually coherent rays that suffices for producing the interference pattern known per se with the aid of the phase grating arranged downstream of the object in the ray direction.
It is possible in this way to use radiation sources that have dimensions corresponding to the normal X-ray tubes in CT systems or transmitted light X-ray systems, although the image resolution continues to be determined by the extent of the focus.